iTerm2 is a macOS terminal emulator with a lot of extra features. In particular, it has a simple protocol for displaying images inline. It comes with a program imgcat that will display common image formats such as PNG, JPEG, GIF, etc. Most of the images I deal with are FITS, though. I wrote fitscat to be a handy utility to display FITS images, as well as to print basic information about the FITS file such as a listing of extensions or an extension header.

For example, fitscat can display an image in an extension, as seen below:

There are options to specify a minimum and maximum value for scaling, as well as to use a simple filter such square root.

Also, fitscat can also print basic information about a FITS file, such as a listing of extensions:

CoMP$ fitscat --list 20150624.170419.comp.1074.iqu.5.fts
Filename: 20150624.170419.comp.1074.iqu.5.fts
No.    Name      Ver    Type      Cards   Dimensions   Format
  0  PRIMARY       1 PrimaryHDU      67   ()
  1  I, 1074.38    1 ImageHDU        33   (620, 620)   float32
  2  I, 1074.50    1 ImageHDU        33   (620, 620)   float32
  3  I, 1074.62    1 ImageHDU        33   (620, 620)   float32
  4  I, 1074.74    1 ImageHDU        33   (620, 620)   float32
  5  I, 1074.86    1 ImageHDU        33   (620, 620)   float32
  6  Q, 1074.38    1 ImageHDU        33   (620, 620)   float32
  7  Q, 1074.50    1 ImageHDU        33   (620, 620)   float32
  8  Q, 1074.62    1 ImageHDU        33   (620, 620)   float32
  9  Q, 1074.74    1 ImageHDU        33   (620, 620)   float32
 10  Q, 1074.86    1 ImageHDU        33   (620, 620)   float32
 11  U, 1074.38    1 ImageHDU        33   (620, 620)   float32
 12  U, 1074.50    1 ImageHDU        33   (620, 620)   float32
 13  U, 1074.62    1 ImageHDU        33   (620, 620)   float32
 14  U, 1074.74    1 ImageHDU        33   (620, 620)   float32
 15  U, 1074.86    1 ImageHDU        33   (620, 620)   float32

Or display a header:

CoMP$ fitscat --header -e 3 20150624.170419.comp.1074.iqu.5.fts
XTENSION= 'IMAGE   '           /extension type
BITPIX  =                  -32 /bits per data value
NAXIS   =                    2 /number of axes
NAXIS1  =                  620 /
NAXIS2  =                  620 /
PCOUNT  =                    0 /
GCOUNT  =                    1 /
EXTNAME = 'I, 1074.62'         /
WAVELENG=             1074.620 / WAVELENGTH OF OBS (NM)
POLSTATE= 'I       '           / POLARIZATION STATE
EXPOSURE=               250.00 / EXPOSURE TIME (MILLISEC)
NAVERAGE=                   16 / Number of images averaged together
FILTER  =                    1 / FILTER WHEEL POSITION (1-8)
DATATYPE=               'DATA' / DATA, DARK OR FLAT
LCVR1TMP=            29.639999 / DEGREES CELSIUS
LCVR2TMP=            33.429001 /
LCVR3TMP=            33.715000 /
LCVR4TMP=            33.738998 /
LCVR5TMP=            33.618999 /
LCVR6TMP=            28.847000 /
NDFILTER=                    8 / ND 1=.1, 2=.3, 3=.5, 4=1, 5=2, 6=3, 7=4, 8=cle
BACKGRND=               13.154 / Median of masked line center background
BODYTEMP=               34.023 / TEMPERATURE OF FILTER BODY (C)
BASETEMP=               33.599 / BASE PLATE TEMP (C)
RACKTEMP=               25.012 / COMPUTER RACK AMBIENT AIR TEMP (C)
OPTRTEMP=               33.306 / OPTICAL RAIL TEMP (C)
DEMULT  =                    1 / 1=DEMULTIPLEXED, 0=NOT DEMULTIPLEXED
FILTTEMP=               35.000 / ILX FILTER TEMPERATURE (C)
FLATFILE= '20150624.070023.FTS' / Name of flat field file
INHERIT =                    T /
DISPMIN =                 0.00 / Minimum data value
DISPMAX =                 5.00 / Maximum data value
DISPEXP =                 0.50 / Exponent value for scaling

The full interface of fitscat is shown below:

$ fitscat --help
usage: fitscat [-h] [--min MIN] [--max MAX] [--debug] [-d] [-l] [-r]
               [-e EXTEN_NO] [-f FILTER] [-s SLICE]
               filename

fitscat - a FITS query/display program

positional arguments:
  filename              FITS file to query

optional arguments:
  -h, --help            show this help message and exit
  --min MIN             min for scaling
  --max MAX             max for scaling
  --debug               set to debug
  -d, --display         set to display
  -l, --list            set to list HDUs
  -r, --header          set to display header
  -e EXTEN_NO, --exten_no EXTEN_NO
                        specify extension
  -f FILTER, --filter FILTER
                        specify filter (default: none)
  -s SLICE, --slice SLICE
                        specify slice of data array to display

Source code for the Python script is available on GitHub. The script is compatible with Python 2 and 3, but requires standard scientific Python packages AstroPy, NumPy, and PIL.